1. Field of the Invention
The present invention relates to a scanning conversion method for converting television signal transmitted by an interlace scanning system into those of a progressive scanning system and an apparatus therefor. More particularly, it relates to a method of and an apparatus for converting a 2:1 interlace scanning system into a progressive scanning system.
2. Description of the Prior Art
There has been discussed in the art provision of an apparatus a method for converting interlace scanning system television video signal into progressive scanning system television video signal, in order to remove various influences such as line flickers caused by the interlace scanning system thereby to facilitate overall improvement of the picture quality.
In an NTSC system television signal, for example, field period signals of 262.5 scanning lines are transmitted by interlace scanning per 1/60 sec., thereby to form a frame picture of 525 scanning lines by two fields. In other words, video signals of two different fields are composed per 1/60 sec., thereby to form a frame picture in the period of 1/30 sec. A scan converter unit is adapted to store the transmitted field period signals at prescribed timings and read the same to convert scanning of two field video signals forming one frame, from interlace scanning into progressive scanning, thereby to reproduce high quality picture identical in field frequency to the interlace scanning system and doubled in the number of scanning lines.
FIG. 1 is a block diagram schematically showing the structure of a conventional scan converter unit. Such a scan converter unit temporarily digitalizes an input video signal to perform writing and reading at prescribed timings, thereby to re-generate the same as an analog signal.
Referring to FIG. 1, video-detected input video signal 1 of the 2:1 interlace scanning system is inputted in an analog-to-digital (A-D) converter 2 to be converted into a digital signal, which is written in a storage unit 3 formed by semiconductor memory or the like. The storage unit 3 has periods for writing input data and those for continuously reading the written data in a time divisional manner. The digital signal thus converted and composed by writing and reading timings is progressively output from the storage unit 3. The digital signal read from the storage unit 3 is input in a digital-to-analog (D-A) converter 4 to be converted into analog video signal 5, to be supplied to a display unit (not shown) implemented by a CRT or the like. Respective timings of the A-D converter 2, storage unit 3 and D-A converter 4 are controlled by a timing signal generator 6.
Although the block diagram showing the conventional structure is illustrated with respect to only one signal system and such a circuit may be applied only in a monochrome television receiver processing a luminance signal alone, the circuit as shown in FIG. 1 must be employed for respective one signal systems where processing of a plurality of signals, formed by a luminance signal and color difference signals or respective primary color signals of red, green and blue, is required by application to, e.g., a color television receiver.
The operation of the aforementioned conventional scan converter unit is now described with reference to FIGS. 1 and 2.
FIG. 2 is a diagram for illustrating timings for writing and reading signals of the storage unit 3. Numeral 7, shown in the upper part of FIG. 2, depicts the waveform of the video signal transmitted by 2:1 interlace scanning, and numeral 8, shown under the same, depicts the waveform of vertical deflection current in the interlace scanning. Shown in the lower part of FIG. 2 are timings at which the storage unit 3 performs writing and reading of signals with respect to the aforementioned waveforms 7 and 8 respectively.
In the operation timings of the storage unit 3 as shown in FIG. 2, the abscissa indicates the time and the ordinate indicates memory addresses of the storage unit 3, while discontinuous step-shaped solid lines 9 are illustrative of the writing timings of the storage unit 3, and continuous dot lines 10 are illustrative of the reading timings of the storage unit 3. Further, symbol 1H indicates one horizontal scanning interval in interlace scanning, symbol 1V one vertical scanning interval in interlace scanning and symbol 1H' one horizontal scanning interval in progressive scanning. It is understood from FIG. 2 that the vertical scanning interval 1V is identical in both of the interlace and progressive scanning systems, while one horizontal scanning interval 1H' in the progressive scanning system is half that in the interlace scanning system.
In FIG. 2, the ratio of the horizontal scanning intervals to the vertical scanning intervals is set in 1:7.5 for convenience of illustration. Therefore, 15 scanning lines form one picture frame of two fields (one frame) in this case.
When the video signal 7 transmitted in a specific field, e.g., that shown in the left-hand side in FIG. 2 is written in the storage unit 3, signals for one scanning line are written in a specific address and a subsequent address for one scanning line is emptied so that the video signal 7 for the subsequent one scanning line is written therein, and such writing operation is repeatedly performed. After the video signal 7 for one field is thus written, the video signal 7 for the scanning lines is sequentially written in a subsequent field (shown in the right hand side in FIG. 2) in an empty address interlaced by one line in which no signal is written in the forward field.
Such reading of the video signal 7 written in the aforementioned manner is continuously performed in order of addresses at a speed twice that of reading (in FIG. 2, inclination of the dot lines 10 showing the reading timings is twice that of the solid lines 9 showing the writing timings, whereby it is recognized that the reading speed is twice the writing speed). Such timings of writing and reading are supplied from the timing signal generator 6, as hereinabove described.
Consideration is now made on the operation of the storage unit 3 in one horizontal scanning interval A in a specific field of the video signals 7 of the interlace scanning system currently being writing. Read in a period a.sub.1 -a.sub.2 in the scanning interval A (period a.sub.1 -a.sub.2 is the first half period of the scanning interval A) is the signal written in an address ADa.sub.1 -ADa.sub.2. The signal written in the address ADa.sub.1 -ADa.sub.2 is that in a writing period indicated by leftward extension of the section ADa.sub.1 -ADa.sub.2, i.e., video signal in a 1H period B in a field one field ahead.
Further, read in a second half period a.sub.2 -a.sub.3 of the scanning interval A is the video signal of the 1H period A currently being writing in the interval A.
Thus, the one field video signal read from the storage unit 3 is formed by alternate arrangement of video signals of two continuous fields in the unit of 1H period. Therefore, pictorial images can be reproduced in progressive scanning by displaying a signal which remains identical in vertical deflection frequency to the progressive scanning system and which is doubled in horiziontal deflection frequency on a CRT or the like. In other words, reproduced in one field is a pictorial image formed by 15 scanning lines, in the case of this prior art example, in a non-interlace scanning manner. Thus, the scanning system is converted from interlace scanning into progressive scanning.
In the aforementioned description, the ratio of the horizontal scanning intervals to the vertical scanning intervals is set in 1:7.5 for convenience of illustration, and hence 15 scanning lines are present in one field upon conversion into the progressive scanning system. It is to be noted that under the current NTSC system, the ratio of the horizontal scanning intervals to the vertical scanning intervals is 1:262.5, and 525 scanning lines are present in one field upon conversion into the progressive scanning system.
Although writing and reading of signals in one horiziontal scanning interval are described as the minimum unit to simultaneously perform writing and reading of the signals in the same storage unit 3, such operations are performed in a time divisional manner in practice.
In the aforementioned prior art example, the storage unit 3 requires X--X' addresses as shown in FIG. 2. In further detail, required as a whole are addresses twice those capable of storing video signal for one field, in order to write the video signal for one field by writing the video signal for one scanning line and emptying addresses in which the signal for one scanning line can be written thereby to write video signals for a subsequent scanning line from the subsequent addresses. The storage unit 3 requires having a capacity which can store video signals for two fields (one frame) as a whole.
An explanation of the scan converter unit, as shown in FIG. 1, is that assuming that sampling is made, in the A-D converter 2, 910 times in one horizontal scanning interval, i.e., that the sampling frequency is four times the color subcarrier in a color television system and the amount of information for one pixel is formed by 8 bits in NTSC system video signals, the amount P of information to be stored for one frame is: EQU P=910.times.525.times.8=467 KB (byte)
whereby the storage unit 3 is required to store an extremely large amount of information.
Therefore, the cost for the storage unit 3 is increased while the storage unit 3 itself is physically made large, and the space occupied by the storage unit 3 is increased upon formation of the scan converter unit.